The royal editor added that Harry and Meghan's celebrity star power with Americans is because of their connection with the royal family.
"It remains to be seen how successful they will be," he explained before saying that Harry "would be leaning on" his former actress wife's success.
"It's been discussed about how many times can Harry wheel out this story of his own mental hardship? His mother dying very young has had a profound effect on him and he is able to speak from the heart on certain issues, but once you've done that... how many more times can you do it?"
"I'm not sure how many strings he has to his bow."
Russell also remarked that the Duchess of Sussex is "very adaptable", but Harry still needs to get used to their new life.
"She's a successful actress, she's had success speaking with the UN, speaking on a wide range of charitable endeavours.
"They could be a tour de force as a collective, but Harry really needs to find his feet.”
New Idea previously reported that the couple, who are parents to one-year-old Archie, are fighting and bickering constantly and Meghan is frustrated at her husband for not trying to fit in with her friends and business contacts.
“She hates that he wears the same raggedy grey polo shirt and baseball cap all the time – especially when he knows she needs him to look sharp to mix among the type of Hollywood elite who will hopefully pay their bills in the form of plum acting roles for her and lucrative speaking circuits for him,” the source said.
“She married a prince, not a hobo, but in her angrier moments she rants that she can’t tell the difference.”